Giving birth to Joey
Mother and Child
Marsupials are an order of mammals, who themselves are a class of vertebrates (i.e. animals which are equipped with a backbone). Mammals, as an order, maintain a constant body temperature and they sustain their young by feeding the child milk which the child sucks from the breast or some other milk-producing gland of the mother. Most mammals are usually born alive after a period of development within the body of the mother; this birth process is unlike that of members of the other classes of vertebral animals, such as birds and reptiles, whose young mostly develop entirely outside of the mother’s body in eggs which the mother has laid. However, there are two classes of mammals that go against this norm. The platypus, natives of Australia and Tasmania, and the spiny anteaters of Australia, Tasmania and New Guinea, lay eggs in the manner more common to birds and reptiles although both species are, in fact, mammals.
Mammalian young are almost always born more or less fully developed; that is to say that they have all those things that they require to survive. Growth to adulthood under parental or other care, with its attendant improvement in knowledge (learnt from parents or caregivers) is all that is left for the young mammal to achieve. This, however, is not the case with marsupials. The young of this class of mammals are born at a much more imperfect stage than those of non-marsupial mammals and are thereafter usually carried in in a pouch on the mother’s body where the baby remains until it completes the development process which most other mammals have completed in the womb. Indeed, the common possession of a pouch by the members of this group of mammals is the basis of the name marsupial, which is derived from marsupium from Greek through Latin, which term means pouch. There are a few marsupials, however, that have no pouch; the real anatomical difference between marsupials and non-marsupials is the fact that the kidney ducts of female marsupials pass between the genital ducts rather than being placed on either side of them as they are amongst non-marsupials.
Kangaroos are large herbivorous marsupials that are native to Australia and the red kangaroo, Macropus rufus, the largest existing species of kangaroo, has a birth process that is nothing if not interesting. Like other marsupials, the female red produces an imperfectly developed baby, known as a joey, which is less than 25 millimeters (1 inch) at birth; when we consider that the adult red kangaroo can measure some 2.5 meters or more (in excess of 8 feet) from the nose to the tip of the tail, this is a difference in size between mother and child that is unique amongst mammals. Once joey is born, this almost infinitesimal baby crawls to its mother’s following a path that mother has licked in her ventral fur from her urinogenital opening to the mouth of the pouch which it enters. Once the joey has entered into the pouch, it attaches itself to a milk-producing teat where it remains for the next six or so months feeding on milk and growing to maturity. Once joey is attached to the teat, it’s grip on the teat is so strong that it can be separated from it only at the risk of injury.
After she has given birth, usually within a few days of the joey’s birth, the mother red kangaroo mates again and another egg is fertilized. Meanwhile, the joey in the pouch continues its growth until, some 200 or so days after it first entered into the pouch, it emerges from it. For several days after, it continues to return to the pouch for food and shelter spending increasingly longer periods outside of it until some 30 or so days after it first emerged it leaves permanently. However, for a while after it has left the pouch permanently, it will continue to stick its head into the pouch to feed on milk from mother. As for the egg that was fertilized just a few days after the joey’s birth, it has remained all this while in a state akin to suspended animation. The fertilized egg having reached the blastocyst stage, a stage where the fertilized egg has developed into a hollow sphere of cells just one cell thick, stops any further development until such a time as circumstance may permit. Such circumstance may come about in one of two ways.
Albino kangaroo with joey at German Zoo, 2014.
The first circumstance occurs when the joey in the pouch has grown up to the stage where it begins to leave mother’s pouch, mother’s body triggers off a process which starts the blastocyst growing once more. By the time that the older baby is ready to vacate the pouch permanently, the new baby is ready to be born to take its place within the pouch. The second circumstance comes about where the joey in the pouch dies either as a result of malnutrition due to drought or because of an accident. If this happens, the mother’s body immediately triggers the restart of growth of the blastocyst and in a matter of 35 or so days after the loss of the first joey, a new joey has been born to replace it. However, it is worth noting that if the first joey’s death has been as a result of drought and if persisting drought makes the survival of any new babies doubtful, mother is able to stop the growth process of the fetus and reabsorb it into its own body. Ain’t Mother Nature just wonderful?
We will recall that even after a new joey has been born and has replaced the first joey as the occupant of the pouch, the first joey will continue to come and suckle for several days to come. This habit does not detract from the quality of feeding that baby brother or sister gets, for mother is able to produce two different types of milk from adjacent teats! The firmly attached baby is fed exactly what it needs for its development while for the older sibling who requires quite a bit more in the way of nourishment is fed milk which contains up to a third (33%) more of proteins and up to four times (400%) more fat; and, because of the fact that the younger joey is so firmly attached to its own teat, there is never any chance of the different kids been fed meals that are inappropriate to their needs.